The present invention generally relates to Schmitt circuits for semiconductor integrated circuits. More particularly, the present invention is directed to a Schmitt circuit for a semiconductor integrated circuit in which threshold voltages of the Schmitt circuit are set arbitrarily when producing the semiconductor integrated circuit by the master slice method.
A Schmitt circuit is used for various purposes such as for comparing amplitudes of signals and for converting a sinusoidal wave into a rectangular wave. An example of a Schmitt circuit is a bistable multivibrator comprising first and second Schottky clamped transistors and resistors. A collector of the first transistor is coupled to a base of the second transistor and an emitter of the first transistor is coupled to an emitter of the second transistor. An input voltage to the Schmitt circuit is applied to a base of the first transistor, and an output signal of the Schmitt circuit is obtained from a collector of the second transistor.
Usually, a third PNP transistor is provided as an input buffer, and the input voltage to the Schmitt circuit is supplied to the base of the first transistor via the third transistor. A first resistor is coupled between a power source terminal supplied with a power source voltage and a connection between an emitter of the third transistor and the base of the first transistor, and a collector of the third transistor is grounded. A second resistor is coupled between the power source terminal and the connection between the collector of the first transistor and the base of the second transistor. A third resistor is coupled between the power source terminal and the collector of the second transistor, and a fourth resistor is coupled between ground and the connection between the emitters of the first and second transistors. A pair of threshold voltages of the Schmitt circuit can be varied by varying the resistances of the first through fourth resistors.
Conventionally, when providing a Schmitt circuit on a semiconductor integrated circuit such as a large scale integrated circuit (LSI), appropriate resistances for the first through fourth resistors are calculated beforehand when designing the Schmitt circuit having a desired pair of threshold voltages. For this reason, the resistances for the first through fourth resistors must be calculated for each different pair of threshold voltages. Hence, there are problems in that the pair of threshold voltages of one Schmitt circuit for the semiconductor integrated circuit are fixed, and Schmitt circuits having different pairs of threshold voltages must be designed independently according to the needs. As a result, the production cost of the conventional Schmitt circuit for a semiconductor integrated circuit is high.